Added Subqueries page

Author: Adez017

docs/sql/SQL- Advance/assets/subqueries.gif renamed to docs/sql/SQL-Advance/assets/subqueries.gif

@@ -400,6 +400,7 @@ HAVING AVG(salary) > (SELECT AVG(salary) FROM employees);
 **When to Choose What:**
 
 **Use Subqueries When:**
+
 - Logic is clearer with step-by-step thinking
 - You need aggregated values for comparison
 - Checking for existence/non-existence (EXISTS/NOT EXISTS)
@@ -534,6 +535,7 @@ WHERE o.order_date >= '2024-01-01';
 ## Conclusion
 
 Subqueries are your tool for asking layered questions - calculate something first, then use that answer to get what you really want. They make complex logic readable by breaking problems into steps. Start with simple subqueries in `WHERE` clauses, then gradually work up to more complex patterns. 
+
 **Remember :** if your subquery gets too complicated, there's probably a simpler way to write it. Keep it clear, keep it tested, and your queries will be both powerful and maintainable.
 
 <GiscusComments/>

docs/sql/SQL- Advance/subqueries.md renamed to docs/sql/SQL-Advance/subqueries.md

@@ -1,4 +1,4 @@
-import type {SidebarsConfig} from '@docusaurus/plugin-content-docs';
+import type { SidebarsConfig } from '@docusaurus/plugin-content-docs';
 
 // This runs in Node.js - Don't use client-side code here (browser APIs, JSX...)
 
@@ -144,6 +144,14 @@ const sidebars: SidebarsConfig = {
             'sql/SQL-joins/self-join',
           ],
         },
+        {
+          type: 'category',
+          label: 'SQL Advance',
+          className: 'custom-sidebar-sql-advance',
+          items: [
+            'sql/SQL-Advance/sql-subqueries',
+          ],
+        },
       ],
     },
     {